Paper No. 7
Presentation Time: 11:15 AM
QUATERNARY SEA LEVEL HISTORY AND COASTAL GEOMORPHOLOGY: WHAT HAVE WE LEARNED IN THE PAST CENTURY?
Sea level history of the Quaternary is recorded on many coastlines in the form of constructional coral reef terraces or erosional marine terraces. For the first half of the 20th century, age determinations of terraces were not possible and it was not known that uplift rates were variable from one region to another. With the advent of uranium-series dating in the early 1960's, we now know that terraces of the same age are found at different elevations on different coastlines, depending on uplift rate. Early studies also showed that on coastlines thought to be tectonically stable, based on the emerging plate tectonic theory, terraces of the same age in widely separated localities are found at approximately the same elevation. This indicated that during the last interglacial period (~130 ka to ~115 ka) global ice volume must have been lower (and sea level higher) than it is today. U-series dating of emergent reef terrace corals also allowed correlation to warm periods in oxygen isotope records of deep-sea cores, dated by the Th-230 excess method. Warm periods of the past identified in both records were also linked to the Milankovitch (orbital forcing) theory of climate change. Studies of the 1960's-1970's were conducted mainly on tectonically stable Pacific Ocean and Indian Ocean islands and Australia and the tectonically active coastlines of Barbados and New Guinea. Since then, coral-bearing marine terraces have also been dated elsewhere in the Caribbean, and along the coasts of North and South America, Japan, and the Mediterranean. Similarity of ages of marine terraces from diverse coastlines around the world shows conclusively that these are, for the most part, records of global sea level changes. In the 1980's, additional geochronological capability became available with the successful use of thermal ionization mass spectrometry for U-series dating. This method reduces the analytical uncertainty of ages by an order of magnitude and allows estimates of the duration of the high-sea stands that formed marine terraces. Further, application of this method permits U-series dating back to at least 500 ka. Current research is focused on the interpretation of magnitudes of sea level rise represented by marine terraces in light of new models showing the importance of spatially variable glacial isostatic adjustment (GIA) processes.